In many industrial flow systems it is necessary to actuate one valve completely and positively before another valve is actuated in order to prevent undesirable mixing or back-mixing of streams which, in different stages of an operating sequence, may pass through the same lines. For example, in boiler installations, it is common to use product steam or air to purge liquid fuel burners upon shut-down, and as an atomizing fluid in air or steam atomized burners. If, because of imprecise timing of the actuation of valves controlling steam or air flow and fuel oil flow, oil gets into the steam piping system or condensate gets into the fuel supply, a most undesirable and even dangerous situation can result.
In the past, 90.degree. ball valves have been ganged to a single actuator in an effort to operate all necessary valves simultaneously, but this produces the objectionable "cross-over" or "blending" outlined above. The standard valve port openings are so large that both valves of a pair are partially open for some period of time, even when an effort is made to operate them simultaneously. Furthermore, there is a possibility that jamming or other failure of the actuator part way through a stroke may leave both oil and steam valves open for lengthy time periods. Separate actuators and separate electric or pneumatic operators and control systems for valves result in complex equipment arrangements, are subject to failures, and have no positive mechanical interlocks to provide assurance that valve sequencing is properly accomplished.